This invention relates generally to the manufacture and repair of printed circuit boards and more particularly relates to an apparatus and method for retaining heat at a pin-in-hole (PIH) rework site of a printed circuit board during solder fountain rework.
As the complexity of today""s printed circuit boards (PCBs) steadily increases, there is a corresponding increase in the manufacturing costs of producing the printed circuit boards. As a result, the high value of many printed circuit boards demands that they be repaired, wherever possible. In many instances, even less expensive printed circuit boards require repair (i.e., rework), because just-in-time manufacturing and tightly controlled production runs leave little room for shortage.
In a typical pin-in-hole printed circuit board rework procedure, a connector site to be reworked on the printed circuit board is fluxed to clean it by removing oxides prior to soldering. Fluxes consist of natural or synthetic rosins and chemical additives called activators, which remove oxides and keep the rework site clean during soldering. Reliable solder connections can only be accomplished with truly cleaned surfaces. Solvents could be used instead of fluxing to clean the surfaces prior to soldering, but are insufficient due to the rapid rate at which oxides form on the surface of heated metals.
After the connector site has been fluxed, the printed circuit board is placed in a solder fountain rework oven for preheating. The requirements of temperature and time for preheating depends on the printed circuit board construction, age, and exposure to the atmosphere. Preheating the printed circuit board serves several purposes. Preheating the board drives out volatile substances and/or moisture from the board which may cause expansion or delamination in the board when the board is rapidly heated. Preheating the board also prevents thermal shock to the board. Additionally, preheating allows pre-expansion of the printed circuit board prior to soldering. Finally, preheating raises the temperature of the printed circuit board and the component to be removed, enabling quicker component removal.
Next, a solder fountain system is turned on and the solder within the solder fountain system is allowed to reach the proper solder temperature. Solder is a metal alloy, typically made by combining lead, tin, and sometimes indium in different proportions. The proper solder temperature is a function of the proportions of elements used to form the solder. When hot solder contacts a copper surface, a metal solvent action takes place. The solder dissolves and penetrates the copper surface. The molecules of solder and copper blend to form a new alloy that is part copper and part solder. This solvent action is called wetting and forms the intermetallic bond between the parts.
After the solder fountain system reaches the proper temperature, the preheated printed circuit board is placed in the solder fountain system, and the rework procedure begins. In a typical rework procedure, a number of solder cycles are applied to the component on the printed circuit board in order to reflow and remove the component. Each solder cycle consists of a predefined contact time between the leads of the printed circuit board component and the solder, followed by a separation time. Ideally, the leads of the printed circuit board component should be just immersed and wetted without having the solder wave exerting any upward pressure on the printed circuit board which may damage the board. After the printed circuit board has been repaired, the printed circuit board is allowed to cool before handling. After cooling, the reflow area of the printed circuit board is cleaned and inspected for signs of damage.
Once solder fountain rework has been initiated on the printed circuit board, there is a fixed number of solder cycles that can be performed on the printed circuit board before the printed circuit board is irreparably damaged (i.e., each solder cycle increases the risk of damage to the plated through hole (PTH) and the laminate). Thus, it is desirable to perform the rework in the fewest number of solder cycles possible. The effects of solder cycles performed on the printed circuit board are cumulative during the remaining life of the board. Thus, if the printed circuit board is immediately reworked after manufacturing with a large number of solder cycles, the printed circuit board may be irreparable if a rework is required years later.
An important factor contributing to the number of solder cycles required to rework a connector site on the printed circuit board is the heat dissipation that occurs at a rework site during the rework procedure. During the localized rework of a component/connector site, the balance of the printed circuit board acts as a radiator, effectively drawing thermal energy away from rework site. As a result, additional solder cycles are required at the rework site in order to achieve the required reflow temperatures.
Preheating the entire printed circuit board in a solder fountain rework oven prior to rework initially provides some reduction in the flow of heat away from the rework site, but the printed circuit board quickly returns to an ambient temperature shortly after it is removed from the solder fountain rework oven. Thus, there is a need for an apparatus to minimize the dissipation of heat from the rework site of the printed circuit board during the rework procedure.
These and other objects, features and advantages of the present invention will be further described and more readily apparent from the summary, detailed description and preferred embodiments, the drawing and the claims which follow.
The present invention provides method and apparatus for retaining heat at a pin-in-hole rework site on a printed circuit board during solder fountain rework of the board. A preheated heat retention plate is placed in thermal proximity to the rework side of the printed circuit board. In one embodiment, heat retention plate is placed in direct contact with the rework side of the printed circuit board, and is held in place by heat resistant tape. In an alternate embodiment, a spacer is placed between the heat retention plate and the rework side of the printed circuit board, thus preventing direct contact between the heat retention plate and the back side components of the printed circuit board.
The heat retention plate transfers heat to a substantial portion of the board surface during the rework process. Thus, during rework of the printed circuit board, the heat retention plate minimizes the temperature gradient between the rework site of the printed circuit board and the remainder of the printed circuit board. During the rework, the heat retention plate can be heated by an active heater in order to maintain a constant temperature gradient between the rework site of the board and the remainder of the board. By minimizing the escape of heat from the rework site, the number of solder cycles required to rework the board is decreased, resulting in reduced board rework times and increased board longevity.